A conventional gear unit is configured by including a high-speed-side pinion that is fixed to a pinion shaft provided parallel to an axle; a low-speed-side large gear that is fixed to the axle, formed with a diameter larger than a diameter of the pinion, and meshes with the pinion; and a gear case that accommodates therein the pinion and the large gear. This gear unit with the axle is installed on a truck frame, and transmits a rotational torque from a traction motor to the axle to rotate wheels mounted on the axle. The pinion and the large gear are held in the gear case by bearings located on each of both sides of these gears in order that their axial lines are parallel to each other, and mesh with each other. As the bearing of this gear unit, a tapered roller bearing capable of supporting a radial load and a thrust load, and having a large allowable load capacity is used. This bearing is accommodated in a bearing cap fitted to the gear case for ease of maintenance. At the bottom of the gear case, a necessary amount of lubricant oil is stored. The height of the lubricant oil level is managed in order that a part of the large gear is dipped in the lubricant oil.
The lubricant oil stored at the bottom of the gear case, is drawn up by rotations of the large gear and is supplied to a meshing part between the large gear and the pinion (hereinafter, “meshing part”), to low-speed-side bearings provided on each of both sides of the large gear, to high-speed-side bearings provided on each of both sides of the pinion, and the like.
To be more specific, first in the meshing part, lubricant oil sticks to tooth flanks of the large gear and therefore this lubricant oil is directly supplied to the meshing part when the large gear rotates. Next, in the low-speed-side bearing, lubricant oil drawn up by the tooth flanks of the large gear and splashing within the gear case, is collected by an oil pan provided at the top of the gear case. The collected lubricant oil is supplied into the bearing from a small-diameter-side end surface of a tapered roller (hereinafter, simply “roller”) in the low-speed-side bearing. Next, the high-speed-side bearing is explained. Because the pinion rotates at high speed, it is necessary to directly supply lubricant oil particularly to an area where a large-diameter-side end surface of a roller of the high-speed-side bearing (a pinion-side surface of the roller) comes into contact with a large-diameter-side flange portion of an inner race of the high-speed-side bearing.
For example, in a typical conventional gear unit in Patent Literature 1 mentioned below, a pinion and a larger gear are accommodated in a semi-hermetic gear case with these gears meshed with each other, as shown in FIG. 1 of the Patent Literature 1. The lubricant oil in the gear case is drawn up by rotations of the large gear. Further, in this conventional gear unit, an opening provided on the gear side between an inner race and an outer race of a high-speed-side bearing is arranged schematically on the lateral side of a meshing part. That is, the opening of the high-speed-side bearing extends in the width (thickness) direction of each gear and is located close to the pinion with respect to an intermeshing pitch line.
With such a configuration, the lubricant oil sticking between teeth formed on the large gear, is pushed out to both sides of the meshing part (in the width direction of each gear) by the meshing of the large gear and the pinion. The lubricant oil, having been pushed out, enters into the high-speed-side bearing from the opening of the bearing, and is supplied to a contact portion between the large-diameter-side end surface of the roller and the large-diameter-side flange portion of the inner race. As described above, in the typical gear unit according to the conventional technique in Patent Literature 1 mentioned below, the opening of the high-speed-side bearing is arranged schematically on the lateral side of the meshing part. Therefore, lubricant oil is continuously supplied to areas where lubricant oil is required, and overheating and seizure of the bearing are suppressed.